Hi Madeleine,
You can absolutely link park tickets purchased from a third party supplier and a stay at a Disney Resort hotel on your
My Disney Experience account.
It makes no difference whatsoever if you booked your hotel stay and park tickets direct or via a third party. Likewise, it also doesn't matter if your Resort hotel stay and park tickets were bought at different times.
All you need to do is to log onto My Disney Experience and link your Disney Resort hotel reservation and park tickets.
To do this, you will need your Resort Reservation Confirmation Number, last name and 12-digit ID number (or Will Call Confirmation Number) on your park tickets.
The procedure is fairly straight forward but if you do encounter any problems, please don't hesitate to ask me another question.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma